Imaging radiation detectors, for example for X-rays, electrons or generally ionizing radiation, can take the form of a hybrid, in which a read-out chip includes electronics for reading out and processing data, and a sensor is mounted directly over the read-out chip, this assembly having a number of pixels resulting from a regular grid of interconnection bonds between the sensor and the read-out chip (planar geometry).
In an alternative setup the sensor itself has pixels due to a regular arrangement of electrodes machined into the sensor (3D geometry). The pixels are to detect X-rays or electrons or generally ionized radiation, spatially resolved, arriving at the respective pixel. When an X-ray, electron or generally ionizing radiation interacts with a pixel, it creates electric charge which is read out by the read-out chip. Such detectors can act as imaging sensors. See for examples of this approach U.S. Pat. No. 5,889,313 and U.S. Pat. No. 6,204,087.
A commercial radiation detector of the first type is the PIXcel (trade mark) detector sold by Panalytical BV which uses a Si sensor. Each pixel of the sensor has a respective bump bond connecting the pixel to a respective read-out circuit.
When an X-ray is absorbed in a pixel of the sensor it interacts with an atom to produce a photoelectron that in turn excites a number of outer electrons from neighbouring atoms and hence creates a cloud of electrons (or holes) in a small spatial region of the sensor material. There may be of order a couple of thousand electrons in the cloud. The number of electrons is proportional to the energy of the X-ray photon. The bias voltage causes the cloud to diffuse to the back of the sensor where it is passed through the bump bond to the read-out chip and is transformed into an electrical pulse by the individual circuit of the pixel and detected as a count.
The read-out chip can process the electrical pulse created to build up an image. This can be done in a number of ways. Usually counts are collected and spatially resolved, in the pixel matrix of the detector once a generated electrical pulse exceeds a voltage threshold. The different amounts of counts collected in the pixel matrix form an absorption contrast pattern, i.e., an image of an object. In X-ray diffraction experiments the counts of pixels form 2-dimensional X-ray diffraction patterns and they can also be specially integrated or binned together to form diffractograms the latter being realized with the PIXcel detector.
Another option for pixel detectors is for example, that by detecting the size of the pulse in an energy window, and hence the number of electrons generated in the cloud, the energy of the photon can be determined so the approach can in principle be used for energy-dispersive X-ray imaging.
Germanium is used as a radiation detector for high-energy (nuclear) applications, typically using very thick germanium sensors.
The use of germanium instead of Si in a hybrid radiation detector for lower energy radiation, for example 5 to 20 keV, is however made much more difficult because of the absence of good processes for processing germanium. In particular, both contacts to germanium and bump bond contacts are required. The applicants are not aware of any suitable conventional ohmic contact processes in the public domain, though ohmic contacts to germanium are available from some providers who provide this service using proprietary processes. There does not appear to be any known process at all for reliably bump or flip-chip bonding to germanium.
One approach to the use of germanium in a radiation detector is taught by U.S. Pat. No. 6,933,503 which describe a bulk germanium X-ray detector sandwiched between two thin oppositely doped GaAs layers which accordingly constitute a pn diode. One of the GaAs layers is divided to make a plurality of pixels. The germanium is much thicker than the GaAs layers so that the X-rays are absorbed in the bulk germanium.
The patent suggests that the materials system is suitable for a number of reasons, including the similarity of lattice constants between Ge and GaAs, as well as the minimal recombination at the interface.
However, the Ge/GaAs materials system proposed by this patent is very difficult to implement for building hybrid pixel detectors in practice.
Although the field of GaAs pixel assemblies including bump-bonding has been researched for more than a decade, the material is still very crucial and problematic for general performance. This is not only an issue of the bulk material itself but an important aspect is, despite all efforts, still the bump-bonding process. The cited patent proposes an alternative patterned structure in the thin GaAs sandwich layer to form pixels which, however, is very special and far removed from existing main-stream bonding processes. This makes the proposed radiation detector difficult to manufacture.
There thus remains a need for a practical Ge based imaging radiation detector.